This invention relates to an apparatus and method for providing powdered coating materials to various types of objects. More particularly, the present invention relates to a method and apparatus for feeding fine powdered coating material such as thermoplastic type material to a succession of discrete articles passing by the apparatus and providing for recirculation of powder not ultimately used to form the coating back to the powder feeding apparatus.
Although the present invention has a wide range of potential applications, it has been found to be particularly efficient in depositing powdered coating material onto fasteners in order to ultimately form a self-locking, insulating or masking coating on all or a portion of each fastener.
Self-locking fasteners of the type in which the self-locking characteristic is derived from a coating such as a patch of material adhered to all or a portion of the thread defining surface of the fasteners have proven to be very popular for a wide variety of applications in order to prevent loosening of the fastener due to vibration and the like in various applications. Likewise, demand for thermoplastic masking or insulating coatings on fasteners to protect against subsequent exposure of the fastener to deleterious environments is also increasing.
The prior art discloses various methods and apparatus for applying powdered resilient resin or thermoplastic type material to articles such as fasteners. Required in all of these prior known devices and methods has been the necessity to deliver powdered coating material onto and in the area of the fasteners. Such fasteners have either been preheated and/or are post heated in order to melt the powdered material and fuse it to form a coating on a desired portion of each fastener.
The powder feeding aspect of the manufacture of self-locking fasteners is of critical importance. It is most important to maintain a consistent flow of powdered material that is directed against the fasteners so that the amount of powder that contacts the fastener is substantially identical for successive parts moving past the powder feeding mechanism. This is critical because fasteners so processed to have self-locking elements thereon must meet torque specifications. These specifications are directly related to a proper and consistent application and adherence of the powdered material that ultimately forms the self-locking element on the fastener. Similarly, masking and insulating coatings formed on fasteners from powdered material must exhibit a void-free covering of substantially uniform thickness to be useful.
The powder feed system is also very important because the cost of the powdered material is generally a rather significant element of the cost of production of the coated discrete articles. Therefore it is desirable to precisely control the powder delivery and recirculate powder that is fed to the articles to be coated that is not ultimately retained thereon. In addition, many powder coating materials such as thermoplastics and nylons have a tendency to agglomerate with other particles as a result of the presence of moisture, oils or humidity in the ambient air surrounding the powder feed mechanism. This agglomeration causes an inconsistent pulsing powder flow that can lead to an uneven application of materials to successive articles passing by the powder flow.
In powder feed systems that utilize auger in the delivery of powder coating material, this agglomeration problem can be further compounded. If powder material is introduced to the auger in an agglomerated state the auger can effect a packing action on such material resulting in a delivery of unwanted clumps of material rather than an even flow. An unacceptably wide variation in torque values for self-locking fasteners and voids or inconsistent thicknesses in masking and insulating coatings can result from the problems set forth above.
The prior art contains several less than completely adequate solutions to these problems. One group of known systems for feeding powder material toward discrete articles such as fasteners is described in U.S. Pat. No. 3,498,352 to Duffy. In such systems, a supply of powder particles is introduced into a jet nozzle through which a supply of gas under pressure is introduced that entrains the powder and directs it in a stream of particles out the opening of the nozzle and toward the discrete articles. Such systems permit limited control over the powder introduced into the nozzle. There is a significant cost associated with generating and supplying the volume of pressurized air required on a continuous basis by such coating systems. In addition, the powder introduced into the nozzle in such systems is many times rather uneven since the powder particles that are deposited into the nozzle often suffer from significant random agglomeration.
A second type of known powder feed system for supplying powder to a continuous succession of movably supported fasteners is disclosed in U.S. Pat. No. 3,830,902 to Barnes. These types of systems are beneficial in that they eliminate the need for combining the powdered coating material with an airstream. These systems, however, have drawbacks since the powder flowing from the feeder device is subject to significant agglomeration and the rate of powder flow is very difficult to control. Furthermore, these powder feed systems must also have multiple nozzles that are synchronized with each individual fastener passing by the powder feed system. This discontinuous powder flow leads to an uneven application of material to the fasteners passing by the powder feed system.
Vibrating feeder bowls such as those disclosed in Burgess, U.S. Pat. No. 3,280,964, are also generally known in the art. Most of these feeder bowl constructions, however, are designed to orient and deliver a succession of parts such as fasteners to a device such as a conveyor belt and do not contemplate the delivery of powdered materials.
Vibratory feed systems such as disclosed in U.S. Pat. No. 4,963,392 to Molnar are designed for depositing flock or fiber materials onto the surface of a substrate. This system also requires combining the fibers outputted from the vibratory feed mechanism to be delivered with a pressurized air source which entrains and directs the fibers towards the substrate.
There therefore exists a need in the art for an improved powder coating apparatus and method that provides a continuous, highly controlled, very consistent flow of fine powdered material that is delivered to a succession of articles to be coated such as fasteners without necessitating the combining of the powder flow with a separate airstream to entrain the flow of powder or interrupting the powder flow and indexing it with the flow of discrete articles past the powder feed system.